Thermoset plastic/prepreg and liquid shim material are typically used in aerospace vehicle assembly to eliminate gaps and dimensional differences between two joined parts. Liquid shims are commonly epoxy-based structural adhesive materials that possess high compressive strengths. For gaps wider than ˜3 millimeters, solid shims made out of thermoset plastic or prepreg are used, but in cases where gaps are less than ˜3 millimeters or for wide area fit up, a flowable cured polymeric resin material (a liquid shim) is employed.
Key properties in a liquid shim material involve pot life, cure time, compressive strength, resistance to cyclic fatigue, and optimal rheology for convenient application to vehicle surfaces. Longer pot life allows the assembly of larger components and also aides in cleanup of unused material. Excessively long pot life, however, interferes with production throughput, as the shim typically needs time to reach the final cured state before assembly can continue. A limitation of state of the art liquid shims is the relationship between pot life and cure time resulting in shims with either impractically short working times or slow, inefficient cure times. Preferably, a liquid shim material will have a lengthy pot life combined with a rapid cure time for greatest manufacturing efficiency. In an ideal case, a user activated trigger would aid the transition between these two regimes. Heat may be used to achieve such an on/off transition. However, manufacturing of commercial aircraft involves strict limits on the degree of heating that can be applied during assembly processes (typically <140° F.). As a result, the cure transition must be developed to occur in a more narrow range than many heat activated systems.
Current state of the art epoxy resins (such as liquid shims) are amine epoxy based resins that meet the basic requirements for use as adhesives in aerospace vehicles. These resins show typical cure times of 8-9 hrs at ambient temp and 1.5-2 hrs at 140° F. while possessing a compressive strength of ≥8 ksi @ 2% offset at 190° F. Manufacturing flexibility with such compositions is limited due to the relationship between pot life and cure time. Accelerated cure times typically result in short, impractical pot lives while lengthy pot life compositions slow manufacturing efficiency. Furthermore, attempts at epoxy-thiol based resins thus far do not provide resins with adequate mechanical integrity (i.e., compressive strength) to achieve the performance of the current state of the art systems and meet modern aerospace materials standards.
Therefore, there is a need in the art for methods for forming resins and resins with adequate pot life, controllable curing times to form a resin, and mechanical properties that meet aerospace materials standards.